It is only cool in the case of twin sisters as then really it is just masturbatory acts; twin brothers would be impaling each other probably, so it would be different.
(Well... I guess this is sexist kind of, but follow my point, if the sisters are making out, then that is where it is culturally wrong because they are touching a parallel part that individually they only have one of, if one is making out downstairs with the other one, then that is masturbatory)
Otherwise I say that the way the law is set up is meant to protect children who grow up in families. Otherwise, the law should be lessened due to it being consenting adults. If people want to claim that it is bad for society to let it happen (which is why there are laws, to protect the good of society), then people also have to recognize that there are genetic defects in everyone and in today's society. They should also recognize that contraception is a safer method than it was when those laws were probably passed.